[Nescafe]

Hi,

I know acid chlorides and any anhydride you form are usually very reactive, but why, what makes these species so reactive?

Cheers,

Nescafé.

[Organic_lover*]

Hey Nescafe!

The reactivity of acid chlorides is due the presence of Cl atom on it's structure, besides the carbonyl group (that have a good polarity). The acid chlorides can participate on SN2 reactions and any other reactions where the Cl atom can "go out" from the acid chloride due to a nucleophilic attack on the carbonyl carbon (if you remember, the chloride is a good leaving group).



The anhydride, on the other hand, has it's reactivity due the two carbonyl groups on it's structure by the "same" reason: the difference of polarity of the carbon and oxigen atoms of carbonyl groups allow the attack of some nucleophile.



Hope this examples help you to understand.

See ya \o

[Nescafe]

Maybe you can help me with this. I read that chloride is a better leaving group than an anhydride (when adjacent to a carbonyl, hence the anhydride).

I do not understand why this is so.

Cheers,

Nescafe.

[zs3889]

I guess chloride is more electronegative than oxygen?

[Nescafe]

I guess chloride is more electronegative than oxygen?

Yea, spot on.

[Babcock_Hall]

For compounds that are closely related, leaving group ability correlates better with basicity, not with electronegativity.  For example, in terms of leaving group ability Cl- < Br- < I-.  This is anticorrelated with electronegativity.  However I- is the weakest base of the three, which is another way of saying that HI is the strongest acid of the three.

With respect to acetyl chloride versus acid acetic anhydride reacting with an amine, you can still use basicity to understand reactivity.  Which is the stronger base, Cl- or acetate ion?  The stronger base is usually the poorer leaving group (there are some exceptions when you compare compounds that are very different in structure).  I am not 100% certain that this is what you questions was about, so please let me know if it needs to be clarifed.